Adding `content_security_policy` to a few sample extensions.

chrome/common/extensions/docs/examples/api/pageAction/pageaction_by_url/background.js

+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Called when the url of a tab changes.
+function checkForValidUrl(tabId, changeInfo, tab) {
+  // If the letter 'g' is found in the tab's URL...
+  if (tab.url.indexOf('g') > -1) {
+    // ... show the page action.
+    chrome.pageAction.show(tabId);
+  }
+};
+
+// Listen for any changes to the URL of any tab.
+chrome.tabs.onUpdated.addListener(checkForValidUrl);